Modern manufacturing methods allow for very high production rates of rolled sheet metal products, such as aluminum, brass, and steel. Indeed, modern metal rolling equipment and techniques are capable of rolling sheet metal at thousands of feet per minute. Not surprisingly, monitoring the quality of sheet metal so produced represents a formidable challenge. These very high production rates make unworkable the traditional method of monitoring the manufactured goods, namely having a human inspector look carefully at the surface of the sheet metal (also known as a web) as it rolls off the mill.
The concept of replacing a human inspector for detecting flaws in moving webs with an optical detection system is known. Such optical detection systems typically have a source of light trained on the surface of the web and a detector element for detecting reflected or scattered light. For detecting flaws in rather wide material webs, such as rolled sheet metal, it has been known in the art to use a scanning beam of laser light to traverse the width of the material web.
U.S. Pat. No. 4,511,803 of Ross et al. discloses a fault detection apparatus for material webs in which laser light is scanned back and forth at very high speeds to create a line of light and the light reflected from the surface of the web is filtered with a hologram to allow the apparatus to detect scattered light for the detection of surface faults. Unfortunately, scanning techniques are generally unsatisfactory for inspecting very rapidly moving webs because the scanning equipment, although capable of rapidly scanning back and forth, generally is unable to scan quickly enough to inspect the entire surface of the web without missing some areas. Simply put, the scanning techniques are complex and are not able to keep up with the very rapidly moving rolled sheet metal.
In addition to the above identified problem of ensuring that the entire sheet is inspected, a surface inspection system also faces the task of reliably distinguishing defects from good surfaces and distinguishing both the type and magnitude of the defect. Furthermore, the environment of a typical metal rolling plant is harsh, particularly in the area of the rolling mill itself, with the rolled sheet metal often being hot and giving off oily fumes.
Accordingly, it can be seen that a need remains for a method and apparatus for surface inspection, particularly inspection of rolled sheet metal products, which is able to inspect the entire surface of the sheet metal despite very high production rates, and which is able to reliably distinguish the existence, type, and magnitude of a defect, while doing so in the harsh environment of a metal rolling plant. It is to the provision of such a method and apparatus that the present invention is primarily directed.